Terry Pratchett’s first Discworld novel was published in 1983. As a wartime child in the 1940s I was already puzzling over an image of a domed world poised on the backs of three elephants that stood on a monstrous turtle. This discworld had a small temple on top of it, and the clawed feet of the turtle rested on the coils of a huge serpent, which also stretched to encircle the world, with the point of its tail in its mouth. It was reproduced in my favourite book, Asgard and the Gods, a scholarly German work on Norse myths, which my mother had used at Cambridge.

This image, and this book, provoked my earliest thinking about the nature of belief and its relation to storytelling. Where on earth did the idea of the turtle and the elephants come from? Did people really believe in them? These questions were related to the kind of embarrassed pain with which I contemplated the stories of origins I was expected to believe in, the Bible with its heaven and hell, the tale of judgement to come.

Pratchett’s new book, The Science of Discworld IV, co-written with the mathematician Ian Stewart and the biologist Jack Cohen, discusses ideas about origins and endings, cosmology and astrobiology, entropy and genetics. The idea of storytelling is not just an embroidered way of including a tale of the discussion of the “Roundworld” taking place on the Discworld. Human beings are defined as pan narrans, the storytelling ape, who exists in a dimension known as the “narrativium”. We look for causes because we think in linear sequences of words. We look for origins because we arrange our world into narrative strings with beginning, middle and end. Stewart, Cohen and Pratchett set out to puzzle us and make us think differently.

Central to their approach is the distinction made by the physicist and sciencefiction author Gregory Benford between human-centred thinking and universecentred thinking. Human-centred thinking comes naturally to human beings. “In this world-view, rain exists in order to make crops grow and to provide fresh water for us to drink. The sun is there because it warms our bodies.” From human-centred thinking comes the idea of a ruler of the universe, as well as the idea that the earth and the creatures, the sea and the oil and the forests are somehow there for our benefit. Universe-centred thinking, on the other hand, sees human beings as “just one tiny feature of a vast cosmos, most of which does not function on a human scale or take any notice of what we want”. The universecentred thinker must have what Keats called “negative capability” – the capacity to be in uncertainties, mysteries, doubts, without any irritable reaching after fact and reason. This is hard and invigorating.

The writers discuss creation myths – including a number of myths about cosmic turtles and scientific ideas about origins, including the Big Bang. They discuss the evolution of ideas about evolution, recent thoughts about the relation of RNA to DNA and the idea of the curvature of space. They also consider neural networks and decision theory, and the strong and weak anthropic principles – ideas about how the physical universe is uniquely suited to the existence of human beings.

Pratchett and co also explore the psychology of belief and disbelief. They describe one way of coming to conclusions – the brain taking in new evidence, and fitting it to the knowledge and beliefs it already holds. This is what they call “System 1”, and it includes scientific thinkers as well as people with inherited religious beliefs. There are, they say, scientists who “know” that DNA is the most important part of the system, physicists who “know” that the world is moving towards entropy. “System 2”, on the other hand, is steadily analytical and sceptical – “trying, not always successfully, to ignore inbuilt prejudices”. Karl Popper’s system of “critical rationalism” held that a theory could be considered scientific if, and only if, it was capable of falsification. Stewart and Cohen claim that “scientists actively try to disprove the things they would like to be true”. They use the example of a believer in UFOs who sees disbelief in UFOs as another form of belief. “Zero belief in UFOs”, they point out, is not the same as 100 per cent belief in the non-existence of UFOs. “Zero belief is an absence of belief, not an opposed form of belief.” What they aspire towards and desire is “a disbelief system”. This is exhilarating.

I remember being on a platform where various poets and writers discussed the ways in which the arts could figure the world of the scientist. That blunt sceptic Lewis Wolpert, sitting in the audience, rose to inform the assembled artists that we would not understand any of his work were we to find ourselves in his laboratory. Some of us were indignant but I believe he was right. People like me can read what is written by those scientists who try to tell us about neurons, genes, the shape of the brain, the shape of space and time. We can respond to those descriptions but we are responding to stories, at second hand.

One of the most pleasing things about Pratchett, Stewart and Cohen’s book is the way the authors demonstrate that we don’t understand even what we think we understand. I realised, reading their account of the complex relations between RNA and DNA, that I had been guilty of holding a belief. I was very excited in the late 1970s by ideas about the “selfish gene”, and particularly by the points made by John Maynard Smith about the immutable nature of the inherited and eternal germ cell. Now the New Scientist is full of articles about newly discovered “orphan DNA”. Stewart and Cohen write:

Darwin’s tree of life, a beautiful idea that derives from a sketch in The Origin of Species and has become iconic, gets very scrambled around in its roots because of a process called horizontal gene transfer. Bacteria, archaea and viruses swap genes with gay abandon, and they can also insert them into the genomes of higher animals, or cut them out. So a gene in one type of bacterium might have come from another type of bacterium altogether, or from an archaean, or even from an animal or a plant.

The story I believed in has to be modified and rethought. When I read this, I think in a human way with a series of images, in the grammar of a story. I should not be able to recognise any gene, let alone think intelligently about it. Stewart and Cohen are very good at illustrating our incapacity to understand. They do so with images and stories. My favourite is the one they tell to make us think about the difference between complicated chemistry and the “organised complexity” of the ribosome. It is a story about caramel.

Every cook knows that heating sugar with fats, two fairly simple chemical substances, produces caramel . . . Caramel is enormously complicated on a chemical level. It includes innumerable different molecules, each of which has thousands of atoms. The molecular structure of caramel is far more complicated than most of the molecules you’re using to read this page.

But the complexity of caramel, or other complicated polymers, doesn’t produce organised complexity, as ribosomes do. Wolpert would rightly tell me that I still don’t know anything about the ribosomes. But I am at least able now to think about the problem. And the juxtaposition of caramel and brain is unforgettable. There are delights like this on most pages of this book.

In a chapter entitled “Where did that come from?” we are invited to reflect on how we can’t think about things like the origin of an oak tree, or a child, or even a thunderstorm. They make the reader imagine thinking about clouds, the constituents of the atmosphere, static electricity, physics and physical chemistry. Most of us, they say, “will not have come across one or more phrases such as ‘saturated solution’ or ‘particle carries a tiny electrical charge’. These phrases are themselves simplifications of concepts with many more associations, and more intellectual depth, than anyone can be expected to generate for themselves.” Human beings tend to retreat from uncertainty or difficulty into belief stories, like the American Republican candidate who opposed any regulation of the markets on the grounds that this was “interfering with God’s plan for the American economy”.

Pratchett, Stewart and Cohen use their method of complicating descriptions and explanations to examine several problems with things I have trouble with believing myself, because they feel to me like human stories that tidy up our relation to the universe – the Big Bang, the existence of dark matter, entropy and the “anthropic principle”. They discuss conflicting views of the expanding universe and the steady state and cast doubt on the existence of dark matter. They are not propounding or supporting any particular theory of the shape and origins of the universe, but are rather considering evidence that complicates the explanations we have become used to. They are good at picking out the operations of what they call our “very parochial” minds, which use ideas of space and time that evolved with us. “Our view of the universe may be just as parochial as the world-bearing animals of ancient cultures were. Future scientists may view both the Big Bang and four elephants riding on a turtle as conceptual errors of a very similar kind.”

In Richard Feynman’s The Character of Physical Law, they find a tendency evident in “too many physicists” to consider physical reality to be all of reality. Feynman, they write, states that “the same kind of atoms appear to be in living creatures as in non-living creatures [sic]; frogs are made of the same ‘goup’ as rocks only in different arrangements”. Things in the biological world are the results of the behaviour of physical and chemical phenomena with no “extra something.” Stewart and Cohen agree about the “no extra something” but think that a bleak view of the world of particles and elements misses out the complexity of living things, and the things they make and use and learn from.

Entropy may not be our destiny – they see Feynman rather as Pratchett sees his undifferentiated auditors, who want to tidy everything up into packets of particles. Life, say Stewart and Cohen, has “lifted itself up into a story”. That is a metaphor – and an attractive one. It feels right, and should therefore be regarded with the necessary doubt and suspicion. They also take a mocking run at the idea of “fine-tuning”, the idea that the world has evolved as the only possible world in which humans could exist – just the right amount of carbon and water, and so on.

This “anthropic principle”, in both its strong and its weak forms, has always horrified me because it is so clearly a function of the human mind thinking in a humancentred way. Isn’t it amazing, say the Discworld scientists, that our legs are just long enough to reach the ground? Isn’t it amazing that there was a hole exactly the right size to contain that puddle? And what about a sulphur-centred form of thought?

There was one chapter I found hard to understand – on the curvature of space, round worlds and disc worlds. This was where I wished the book had illustrations – I read pages about the doughnut-shaped torus, and then had the sense to consult Wikipedia, where I could see what was being discussed. And I also needed to see the geometry of the wonderful Escher image of angels and demons.

I have become rather sad about surviving into the anthropocene age of human history, where everything is controlled and constructed by and for what the King of Brobdingnag called “the most pernicious race of little odious vermin that Nature ever suffered to crawl upon the surface of the earth”. But, paradoxically, both Pratchett’s storytelling and the resolutely universecentred perspective of the scientists make me happier to be human. I look forward to the next volume.

“The Science of Discworld IV: Judgement Day” by Terry Pratchett, Ian Stewart and Jack Cohen is published by Ebury Press (£18.99). A S Byatt’s most recent book is “Ragnarok: the End of the Gods” (Canongate, £7.99

Both A S Byatt and Terry Pratchett will be appearing at How the Light Gets In, the festival of philosophy and music in Hay on Wye. For more details, visit: www.howthelightgetsin.org